It is well known that the electrical resistance of a conductor varies with its dimensions. For example, if a conductor is stretched along the direction of the electrical current flow, the current path is increased while the cross-sectional area through which the current flows is reduced by the Poisson's ratio. As a result, the electrical resistance increases. This phenomenon has been widely adopted in strain gauges. In U.S. Pat. No. 2,457,616, a foil type strain gauge is described in which a thin zigzag strip of conducting material, such as, metal, is embedded in a thermoplastic foil. The foil is rigidly bonded onto a strained surface. The varying strains are coupled to the conducting material and can be inferred from the changes in the measured electrical resistance of the conducting material.
The gauge factor of a strain gauge is commonly referred to as the ratio of the relative change in electrical resistance to the relative change in length, the latter of which is defined as the mechanical strain. There exists a class of materials in which the electrical resistivity changes with strain. This is known as the piezoresistive effect. In strongly piezoresistive materials the effect can produce a gauge factor that is much larger than that attributable to geometrical changes. Foil strain gauges of either metal or alloy type are not particularly piezoresistive. Their gauge factors are rather small, which therefore results in low sensitivity that affects accuracy. Furthermore, metals and alloys typically have rather low yield strengths and are prone to fatigue. Thus they may suffer from hysteresis errors after repeated strain cycling.
Another common type of strain gauge is of the semiconductor type, for example, those which employ single crystalline silicon as the conducting material. Silicon is a superb mechanical material with high ultimate strength and is perfectly elastic up to the fracture point. Moreover, single crystalline silicon is strongly piezoresistive, resulting in a gauge factor that is typically tens of times higher than those in metal or alloy strain gauges. Despite these advantages, silicon strain gauges do have the drawbacks for being temperature sensitive, nonlinear and fragile. Moreover, single crystalline silicon is an anisotropic material in which the piezoresistive effect is directional. As a result, the electrical resistance of a silicon conductor is not only sensitive to the strain in the longitudinal direction along which the electrical current flows; but also to the transverse and shear components of strains as well. This will generate significant crosstalk and measurement errors in silicon strain gauges if not corrected.
The majority of sensors in use today are of the micro-electro-mechanical systems (MEMS) type. MEMS based sensors are typically realized with silicon micromachining that originated from integrated circuit fabrication and still shares many of its processing technologies. In addition, there are a few unique processes specifically tailored toward the fabrication of 3-dimensional microstructures. These include double-side photolithography, Deep Reactive Ion Etching (DRIE), and wafer bonding to name a few. With demonstrated advantages that include low cost, small size, high accuracy, high reliability, and high stability, MEMS sensors have become the dominant type of sensors in use for automotive, medical, industrial and consumer electronics applications. An example of an MEMS strain gauge with multiple piezoresistive sensing elements is described in A. A. S. Mohammed, W. A. Moussa, and E. Lou, “Development and Experimental Evaluation of a Novel Piezoresistive MEMS Strain Sensor,” IEEE Sensors Journal, vol. 11, no. 10, pp. 2220-2232, 2011. Of particular interest are the surface trenches that are etched in the vicinity of the sensing elements to reduce (but not completely eliminate) crosstalk and to create stress concentration regions where sensor sensitivity is enhanced. However, due to the limitation of PN-junction isolation, this strain gauge is not capable of operating in environments above 150° C.
Accordingly, in the field of strain gauges, a need presently exists for an improved MEMS strain gauge which is capable of operating in high temperature environments and can measure uniaxial and biaxial strains without being affected by crosstalk.